Epitaxial technology where heavily arsenic (As)-doped semiconductor acts as substrate has been increasingly widely used in device fabrication, and especially in diode, triode, VDMOS, varactor diode, IGBT and etc. With the miniaturization and the increase of integration level of devices, the consistency of epitaxial wafer having heavily As-doped silicon substrate and the transition region width of epitaxial layer are crucial for the performance and reliability of devices.
During chemical vapor epitaxy, the impurity of As is unavoidable between the epitaxial layer and the heavily-doped region of substrate and between the epitaxial layer and the undoped region of substrate. Take the example of the growth of intrinsic epitaxial layer on silicon wafer having heavily-doped region, wherein an impurity distribution vertical to heavily-doped region is named as vertical auto doping (as the direction indicated by AA′ in FIG. 1) and an impurity distribution not vertical to heavily-doped region is named as horizontal auto doping (as the direction indicated by BB′ in FIG. 1), wherein the impurity vertical diffusion mainly comprises two parts of 1. solid thermal diffusion at the interface between the epitaxial layer and the heavily-doped substrate and 2. auto-doping of the impurity ion absorbed by the substrate surface or the impurity gas in the background atmosphere into the epitaxial layer during epitaxial growth, and wherein horizontal auto doping effect is mainly caused by the second phenomenon.
Conventional silicon wafer epitaxial process of heavily As-doped substrate mainly utilizes the process of so-called “two-step epitaxy”, wherein key steps are as follow: 1. loading the substrate in the reaction chamber with the temperature increased to 1000-1200° C. before the hydrogen chloride (HCl) is introduced to clean the substrate surface and the inner wall of the reaction chamber; 2. introducing a large amount of hydrogen (H2) to clean the inner wall of the reaction chamber and the substrate so as to remove the impurity absorbed on the substrate surface and in the reaction chamber; 3. growing an intrinsic epitaxial layer to prevent further out-diffusion of impurities from the substrate; 4. re-introducing a large amount of H2 into the reaction chamber to clean the inner wall of the reaction chamber and the substrate so as to remove impurities absorbed on the substrate surface and in the reaction chamber; and 5. performing a second stage growth until a desired thickness of the epitaxial layer is reached. Conventional process of “two-step epitaxy” has the advantage of minimizing the vertical diffusion effect for thick epitaxial layer growth, and the disadvantages of 1. poor inhibition of vertical auto doping effect for thin epitaxial layer growth; and 2. no obvious inhibition of horizontal auto doping effect. Auto-doping effect of As for epitaxial film formed by conventional chemical vapor deposition is as shown in FIG. 2.
Therefore, it is a technical problem for urgent solution to prevent vertical and horizontal diffusion of atoms heavily doped and reduce the doping concentration for epitaxial layer in undoped region during the epitaxial layer growth on substrate having heavily-doped region.